Laurie Anne Walden, DVM

Roundworms are some of the most common internal parasites in dogs and cats. They can also infect humans. According to the Centers for Disease Control and Prevention (CDC), 13.9% of people in the United States have antibodies to roundworms, meaning they have been exposed to the parasite at some point in their lives.

How dogs and cats are infectedAlmost all puppies are born with roundworms. The type of roundworm that most often infects dogs, Toxocara canis, transfers from a mother dog to unborn pups through the placenta. T canis can also pass to puppies through the mother’s milk. Infected animals excrete roundworm eggs in their stool, so dogs can be infected by eating feces or swallowing roundworm eggs in the environment. Dogs can also become infected by eating a small animal (like a rodent) that is carrying roundworms.

The most common roundworm in cats is Toxocara cati. Cats and kittens are usually infected by swallowing roundworm eggs in the environment or by eating an infected animal. T cati does not pass to unborn kittens through the mother’s placenta.

Ingested T canis and T cati eggs hatch into larvae in the intestines. The larvae migrate through body tissues to the lungs, are coughed up and swallowed, grow into adult worms in the intestines, and begin producing eggs that pass into the environment through the feces.

Roundworm larvae can remain dormant in body tissues of adult animals instead of maturing in the intestines. These arrested-development larvae can’t be detected by fecal tests for worm eggs because they don’t produce eggs. Dormant larvae in a pregnant dog can become active and move through the placenta to the pups. In other words, a female dog with a negative test for roundworms can pass roundworms to her puppies anyway.

Signs of infectionInfected animals often have no symptoms at all. Dogs and cats (especially puppies and kittens) with lots of roundworms may develop a potbelly, vomiting, diarrhea, weight loss, or dull coat. Heavily infected animals sometimes vomit worms, which look a bit like spaghetti noodles, or pass worms in the stool.

Treatment and prevention in petsYoung puppies and kittens should receive multiple doses of deworming medication. The Companion Animal Parasite Council recommends deworming puppies and kittens every 2 weeks starting at age 2 weeks for pups and 3 weeks for kittens, continuing until they are about 2 months old, and then beginning monthly parasite preventives. Many heartworm preventives also prevent roundworm infection.

To reduce the chance your pets will be infected, remove feces from the environment and try to keep them from eating rodents or other wild animals. Have your veterinarian regularly test your pets for parasites, and give them parasite preventives all year round.

Infection in humansPeople can be infected by T canis or T cati if they ingest contaminated dirt or feces. Toxocara eggs can survive in the soil for years. Children and people who own dogs or cats have an increased risk of infection, says the CDC.

Many people with Toxocara infection don’t develop serious disease and have no symptoms. But T canis and T cati larvae can migrate through the bodies of humans, as they do in dogs and cats. Larvae that migrate to internal organs (such as the liver) damage these tissues, a disease process called visceral larval migrans or visceral toxocariasis. Symptoms depend on the organs affected. Sometimes larvae migrate to the eye, causing a disease known as ocular larval migrans or ocular toxocariasis. People with this condition may develop retinal inflammation and vision loss.

Prevention in humansThe CDC recommends these steps to prevent toxocariasis:

Consult your veterinarian for a plan to keep your pets free of parasites.

Practice good hand hygiene: wash your hands with soap and water after handling animals or their feces, after being outdoors, and before touching food.

Keep children away from areas contaminated with animal feces.

Teach children to wash their hands and to not eat dirt.

Keep your pets’ environment clean; bury feces or bag it and throw it in the garbage.